This invention relates to a method for making a printing plate and a method of printing.
Hitherto, numerous printing plates have been developed and practically used.
As light sensitive lithographic printing plates which per se have light sensitivity, there have been known presensitized (PS) plates using diazonium compounds and the like, silver salt type printing plates utilizing silver halide and electrophotographic printing plates using zinc oxide or organic photo-conductive compounds.
As printing plates of high sensitivity and high printing endurance, those comprising an aluminum plate on which an organic photoconductive compound is coated are disclosed, for example, in Japanese Patent Examined Publication Nos. 17162/62, 6961/63, 2426/66 and 39405/71 and Japanese Patent Unexamined Publication Nos. 19509/75, 19510/75, 145538/79, 89801/79, 134632/79, 19803/79, 105254/80, 161863/82, 76843/83, 76844/83, 18658/83 and 244959/85. These printing plates are made through the steps of corona charging, camera exposure or laser exposure, wet or dry-type development, fixation and etching of non-image area with alkaline solution.
However, these plate making methods which use the above mentioned electrophotographic process suffer from some disadvantages and problems, though they have many other advantages. One of them results from use of the corona charging. That is, the mechanism of charging by corona discharge has not yet been clarified and this charging method has the unstableness that the corona charging is greatly affected by changes in temperature and humidity and photographic characteristics and printing characteristics are very dependent on such environmental conditions. Furthermore, when printing plates of large size such as those for newspaper are subjected to scanner exposure by argon, helium-neon or semiconductor lasers, use of corona charging results in difference in charge quantity between the front part and rear part of the printing plate owing to dark decay and thus it is difficult to form uniform toner images overall surface of the plate. Another disadvantage of the conventional method is due to enhancement of sensitivity.
That is, not only in so-called function-separated type printing plate comprising laminated charge generation layer and charge transport layer such as those disclosed in Japanese Patent Unexamined Publication Nos. 118658/83 and 244959/85 which are intended to increase sensitivity, but also in those of single layer type, film thickness of the coated layer is inevitably increased to sustain a sufficient surface charge and furthermore, since organic photoconductive compound is used in a large amount, etching ability of alkaline solution is reduced and as a result, resolving power is lowered or resists superior in printing endurance are difficult to produce.
Moreover, economical disadvantages of the conventional methods are that they are high in cost because expensive photoconductors are coated on printing plates themselves and virtually unable to recover and besides printing plates once used are practically difficult to reuse.
As a result of the inventors' intensive researches considering the various disadvantages and defects seen in the conventional electrophotographic lithographic printing plates, a method for making lithographic printing plates free from these problems has been found.